Rugnux: fit the beam centre without the transmitting beam-stop pieces

a1816e905 adds the partly transmitting holder-arm pieces to the beam-stop mask, and the
background beam-centre fit read that mask too. A thin arm running into the beam, masked, takes
cells out of the rings it crosses; which radial bins stay above the sector-coverage gate then
changes as the trial centre moves, and a walk seeded beside the arm (the FFT capture sat 48 px off)
limit-cycled on that edge and stopped 44 px short (sigma 0.34 -> 2.27 px). On an open-arm set whose
header centre is 171 px out, the measured centre no longer indexed (60/60 -> 0/60), the run kept the
file's centre and merged P1 at CC1/2 0.004 instead of P212121 at 0.995.

The arm's dimming is multiplicative in azimuth, which the fit's per-sector amplitude already
absorbs, so the fit now uses the mask without those pieces (ShadowFinder marks them TRANSMITTING);
integration still masks them. The fitted centre is back to its a0518abe6 value on every set checked
(the failing one plus four arm sets); the failing set merges P212121, CC1/2 0.995, ISa 22.6.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01D1G8gJVAy6gp1K5Dz3NE5C

image_analysis/beam_stop/ShadowFinder.cpp

@@ -831,7 +831,7 @@ std::vector<uint32_t> ShadowFinder::GetMask(size_t nthreads) const {
             region[i] = 0;
 
     for (int i = 0; i < n_pixels; i++)
-        mask[i] = region[i] ? 1 : 0;
+        mask[i] = region[i] ? SHADOW : 0;
 
     // Hardware that lets part of the beam through - a thin holder arm, or one whose shadow is blurred
     // by its distance from the detector - sits between PENUMBRA_RATIO and SHADOW_RATIO along most of
@@ -878,7 +878,7 @@ std::vector<uint32_t> ShadowFinder::GetMask(size_t nthreads) const {
         if (n_dim >= MIN_SHADOW_PIXELS && n_low >= MIN_CORE_PIXELS)
             for (const int i : component)
                 if (dim[i])
-                    mask[i] = 1;
+                    mask[i] = TRANSMITTING;
     }
     return mask;
 }